The present invention relates to a cement admixture for hydraulic cement-based compositions. More particularly, it relates to admixtures for cement-based compositions which drastically improves the workability of concrete, mortar or paste by preventing the decrease in workability with the progression time when the unit amount of water in such a cement admixture is reduced.
Cement compositions, such as mortar or concrete, lose their fluidity with the progression of time due to the hydration reaction of cement and water. This loss reduces its workability. This phenomenon is commonly known as "slump loss". Various proposals have been made to solve this problem.
Some proposals for reducing slump loss are, for example, the addition of a set-retarding agent to fluidized concrete, the use of a granular fluidizing agent, such as beta-naphthalene sulfonate-formaldehyde condensate (.beta.-NSF) [Japanese Patent Publication Kokai No. 139929/1979] and the use of slow-releasing chemical agents [Japanese Patent Publication (Kokai) No. 16851/1985]. However, these methods have poor effect on the prevention of slump loss or have practical problems and, accordingly, they are not effective methods.
Thus, when a set-retarding agent, such as a hydroxycarboxylic acid, is added to a cement-based composition to prevent chemical curing, it is difficult to prevent physical aggregation of the components even if the hydration reaction of cement is delayed. Thus, slump loss may not be prevented. Further, when one uses an amount of the set-retarding agent which does not cause delay of the effect, it cannot cope sufficiently with the situation. In the case where granular fluidizing agent, such as .beta.-NSF, is used, it may show some degrees of the slump loss preventing effect, but the granular agent may not be well distributed within the cement composition and, thereby, remains locally and lowers the strength and durability of the final product. In the case where a slow-releasing chemical agent, such as a water-insoluble copolymer of an olefin and an ethylenically unsaturated dicarboxylic anhydride, is used, it may provide the slump loss preventing effect, but the cement causes the water-insoluble copolymer to precipitate. Such agents are not easily handled and lack storage stability.
In addition to the above, the economics of the cement industry dictates that the components used, including admixtures, be cost-effective.
It is highly desired to have an easily handled, cost effective agent which can inhibit slump loss in cement-based compositions. The present invention, described in detail herein below, provides an admixture for cement compositions which solves the problems described above.